This invention relates to a combustion process for internal combustion engines, in which a fuel-air mixed with back-fed exhaust gas is introduced into a combustion chamber.
In an effort to reduce the emission of harmful substances from internal combusion engines, it has been proposed that engines of this type be driven by a nearly stoichiometric fuel-air relationship and a subsequently added 3-way catalytic converter, in order to achieve the advantage of a simultaneous reduction of the three components of harmful emission, viz., NO.sub.x, CO and CH.
However, this advantage is outweighed by considerable disadvantages. Due to the higher raw emission of the engine, correspondingly high emissions occur behind the catalytic converter, because of the failure to achieve a 100% conversion rate. In case of reduced efficiency of the catalytic converter, therefore, the danger of increased emissions exists. Also, the high thermic demands placed upon the engine may require a reduction of the stated efficiency, and the increased knock danger during operation with a nearly stoichiometric mixture may force reduction of the compression relationship, which worsens the actual degree of effectiveness. Finally, the quality coefficient is also negatively affected by the high process temperatures for nearly stoichiometric mixtures, due to high heat loss through the walls. It has recently been demonstrated that these disadvantages can be partially avoided by additional exhaust gas back-feed, as is set forth by Menne, Stojek and Cloke (VDI Reports #531, pp. 131-150) for the purpose of achieving an additional NO.sub.x reduction. The raw emission of NO.sub.x drops, and the back-fed exhaust gas acts as an inert component to reduce the process temperature so that the compression can be kept at a higher value. This process, on the other hand, has the disadvantage that the back-fed exhaust gas affects the speed of combustion and hence the actual degree of effectiveness. Also, misfiring can occur, which endangers the catalytic converter.
Low exhaust emissions can also be obtained with extreme reduction of the fuel-air relationship (in the range of excess air figures of 1.6 and above). Such a meager mode of operation, however, is only thermodynamically favorable under the condition of a safe ignition and a rapid burn-through. However, a further reduction of the NO.sub.x emission by means of a 3-way catalytic converter is not possible, due to the presence of oxygen in the exhaust. The known procedures for conditioning of the exhaust with ammonia-water mixtures and a subsequently-added catalytic converter (cf. e.g. Sturm, "Molecular sieve as a NO.sub.x catalytic converter", in Sonnenergie und Waermepumpe #2, 1984) enable a selective catalytic reduction of NO.sub.x emissions, but give rise to additional risks (working with NH.sub.3, as well as NH.sub.3 emissions). The currently known process for reduction of emissions of harmful substances in internal combustion engines rely on two contrary concepts for solving the problem:
The generally preferred solution is the measure of post-processing of the exhaust gas after the exit from the engine, and the best current process of this type is exhaust gas processing through so-called 3-way catalytic converters, in which the 3 components of the harmful emission, NO.sub.x, carbon monoxides and hydrocarbons, are simultaneously reduced. This mode of operation, however, requires that the fuel-air relationship of the mixture to be burned indicate values in the nearly stoichiometric range, generally excess-air figures of approx. 0.99-1.01.
This mode of operation, however, has the disadvantage, mentioned above, that high raw emissions occur, and it is particularly disadvantageous in this connection that with reduced effectiveness of the catalytic converter, e.g., due to age or contamination, or with failure of the regulation system, the high raw emissions enter the environment either untreated or inadequately treated.
The opposite concept for solving the problem to the catalytic post-treatment of the exhaust gas in the prevention of the formation of harmful substances through measures designed to affect the combustion within the engine, whereby post-treatment is dispensed with. This approach is selected primarily because a failure of the post-treatment system leads to uncontrolled, relatively high emissions. A possibility for the realization of this concept is to mix exhaust gas into a stoichiometric fuel-air mixture, as mentioned by Menne, Stojek and Cloke (op. cit.) in a comparison of various operational processes for low-emission combustion engines, although they do not see this procedure as particularly advantageous. The admixture of exhaust gas achieves the reduction of the process temperatures and therefore the reduction in the creation of harmful substances. However, due to the limits set for admixture of exhaust gas, even this process cannot meet the high future requirements for freedom from harmful emissions. Also, the extreme dilution of the fuel-air relationship (in the range of excess-air figures of 1.6 and higher) does not permit this goal to be achieved, since catalytic post-treatment is impossible.